Gases give rise to particular hazards so great care must be taken when preparing, collecting or testing

Gases give rise to particular hazards so great care must be taken when preparing, collecting or testing them.

How the gas is to be used will differ from experiment to experiment – it is essential to read carefully the specific instructions given or referred to in the experimental procedure and any accompanying technical notes. This is especially important if the gas needs to be dried.

Gases can be collected by upward or downward delivery or over water. Refer to specific information on each gas below.


In addition to the instructions below, please refer to our standard health and safety guidance and to the CLEAPSS Laboratory Handbook, section 13.2.2.

General gas preparation

The diagram below shows a typical set of apparatus which can be used to prepare a range of gases.

A diagram showing the apparatus required for preparing a range of gases for use in experiments

Source: Royal Society of Chemistry

Typical apparatus used for preparing a range of gases

Gas collection methods

The diagrams below show three different methods for collecting gas.

A diagram showing how to set up apparatus for three different methods of collecting gas, including downward delivery, upward delivery and over water

Source: Royal Society of Chemistry

Three methods of gas collection

Preparing specific gases

Wear appropriate eye protection. The amounts given below are sufficient to generate 1 litre (1 dm3) of each of the named gases.

Carbon dioxide, CO2

Slowly add 42 cm3 of 2 M hydrochloric acid (IRRITANT) to an excess of marble chips. Collect the gas by downward delivery or over water (slightly soluble).

Refer to CLEAPSS Hazcards HC020a and HC047a, plus CLEAPSS Recipe Book RB021.  

Hydrogen, H2

Slowly add 28 cm3 of 3 M hydrochloric acid (CORROSIVE) to excess zinc granules and 1 g of hydrated copper sulfate (HARMFUL). Collect the gas by upward delivery or over water.

Refer to CLEAPSS Hazcards HC048, HC047a, HC107 and HC027c, plus CLEAPSS Recipe Book RB044.

Hydrogen gas is EXTREMELY FLAMMABLE – ensure there are no naked flames.

Oxygen, O2

Slowly add 50 cm3 of 20 vol hydrogen peroxide (IRRITANT) to manganese(IV) oxide powder (HARMFUL). Collect the gas over water.

Refer to CLEAPSS Hazcard HC069, HC050 and HC060, plus CLEAPSS Recipe Book RB064. 


Chlorine, Cl2

Work in a fume cupboard. Method 2 is safer and recommended but slower.

Method 1

Add 14 cm3 of concentrated hydrochloric acid (CORROSIVE) to at least 3 g of potassium manganate(VII) (OXIDISING, HARMFUL and DANGEROUS FOR THE ENVIRONMENT).

Double-check that the acid is hydrochloric and NOT sulfuric. Refer to CLEAPSS Hazcards HC022a, HC047a and HC081, plus CLEAPSS Recipe Book RB024.

Method 2

Add 5 M hydrochloric acid (IRRITANT) to 30 cm3 of recently purchased (10–14% available chlorine) sodium chlorate(I) solution (CORROSIVE) with plenty of stirring. Note that sodium chlorate(I) is only available as a solution often called ‘sodium hypochlorite’; it must not be confused with sodium chlorate(V) (sometimes just called ‘sodium chlorate’), which is a white, crystalline solid. School samples often react too slowly because old sodium chlorate(I) is used. This will have less than the required 10% available chlorine (as it applies to both methods). Refer to CLEAPSS Hazcards HC022a, HC047a and HC089, plus CLEAPSS Recipe Book RB024.  

A diagram showing the equipment required for generating and collecting chlorine gas

Source: Royal Society of Chemistry

The equipment required for preparing chlorine gas

Collect the gas by downward delivery. Chlorine is classified as TOXIC, IRRITANT and DANGEROUS FOR THE ENVIRONMENT.